It is known to join together telescoping tubes with a clamping sleeve which can be tightened to clamp the tubes in a desired relative position. The present invention provides a special arrangement for securing together two telescopible tubes of which the outer tube has an external threaded portion at the end into which the inner tube is to be inserted and the inner tube has an enlarged ring at the end to be inserted within the outer tube, the diameter of the ring being less than that of the bore of the outer tube. According to the invention there is provided these inner and outer tubes, a joining sleeve having an internal threaded portion at one end for engaging the external threaded portion of the outer tube, an internal shoulder facing said one end of the sleeve, an external threaded portion and a jaw at its other end, a clamping sleeve for screwing onto the external threaded portion of the joining sleeve, the sleeve having means to compress said jaw as it is screwed onto the joining sleeve so as to grip an inner tube placed therein and an expandable locking ring located between said shoulder and the end of the enlarged ring facing away from said end of the inner tube and having a normal internal diameter less than the diameter of the enlarged ring and a normal external diameter intermediate the inner and outer diameters of said shoulder.